JP-A-2002-147553, for instance, discloses a belt-type continuously variable transmission for motorcycles, which can steplessly adjust a transmission gear ratio according to a condition of running. This belt-type continuously variable transmission includes a primary sheave, a secondary sheave, and a belt.
The primary sheave is driven by power transmission from an engine. The primary sheave has a pair of clamp surfaces opposed to each other and a belt groove formed between these clamp surfaces. The secondary sheave is interlocked with a rear wheel of the motorcycle via a reduction mechanism. This secondary sheave has a pair of clamp surfaces opposed to each other and a belt groove formed between these clamp surfaces.
The belt is endlessly wound between the belt groove of the primary sheave and the belt groove of the secondary sheave. The belt has contact surfaces for contact with the clamp surfaces of the respective sheaves. Torque of the primary sheave is transmitted to the secondary sheave via the belt by frictional force generated between the contact surfaces of the belt and the clamp surfaces of the respective sheaves.
As is shown in FIG. 18, this kind of belt-type continuously variable transmission has a characteristic that, as thrust, which causes the clamp surfaces of the respective sheaves to clamp the belt, increases, the torque transmissible between the sheaves and the belt increases accordingly. When the thrust acting on the belt increases, a great frictional resistance is generated between the clamp surfaces of the sheaves and the contact surfaces of the belt, and an amount of heat generation of the belt increases. The heat generation of the belt indicates that kinetic energy is converted into thermal energy. The transmission efficiency of torque decreases by a degree corresponding to the conversion from kinetic energy to thermal energy.
FIG. 19 shows transition of an amount of heat generation of the belt and transmission efficiency at the time when the thrust acting on the belt is varied. As it is evident from FIG. 19, if the thrust increases, the amount of heat generation of the belt increases in proportion to the increase in the thrust, and the transmission efficiency of torque decreases. Therefore, it is necessary to set the thrust to a necessary minimum level in order to increase the transmission efficiency of torque between the sheaves and the belt.
On the other hand, in the belt-type continuously variable transmission, the clamp surfaces of the respective sheaves are subjected to machining such as cutting and grinding. This kind of machining is performed while the sheave is being rotated. Therefore, a large number of annular grooves along a peripheral direction are formed on the clamp surfaces of the sheaves. The grooves are very fine with width and depth of about several μm.
Incidentally, according to the conventional belt-type continuously variable transmission, when driving is started in a newly assembled state, slip tends to occur in the belt, in particular, at the initial stage of driving. FIG. 20 shows transition of transmission torque of the belt at the initial stage of driving. As it is evident from FIG. 20, the torque transmitted to the belt is significantly lower than a predetermined set value C immediately after driving is started. A value of this torque tends to gradually increase as driving time elapses. After certain time elapses, the torque reaches the set value.
It is assumed that this phenomenon occurs because of the grooves present on the clamp surfaces of the sheaves in a brand new state. In short, it appears that the presence of the grooves makes a contact state between the sheaves and the belt unstable, causing the slip of the belt.
Therefore, in driving the new belt-type continuously variable transmission, trial-run of the continuously variable transmission needs to be performed until the torque transmitted to the belt reaches the set value. By performing the trial-run, the contact surfaces of the belt are abraded by edges of the grooves of the sheaves and sharp edges of the grooves are worn. Consequently, the grooves of the sheave are filled with abrasion waste and the clamp surfaces of the sheaves are smoothed. As a result, the state of contact between the sheaves and the belt is stabilized and the slip of the belt is controlled. As shown in FIG. 20, desired transmission torque is obtained when predetermined trial-run is completed.
In the conventional belt-type continuously variable transmission, however, the trial-run needs to be continued until the slip of the belt is completely eliminated. Consequently, long time is required until the continuously variable transmission is set in a drivable state and a great deal of labor is required for shipment of the product, causing an increase in cost.
As means for controlling slip of the belt at the initial stage of driving, it is conceivable to increase the thrust acting on the belt. However, if the thrust is increased, the amount of heat generation of the belt inevitably increases as described above. Therefore, after the completion of the trial-run, the thrust acting on the belt becomes excessively large and the transmission efficiency of the torque is deteriorated.